A new kind of traffic detour is coming to the Highway 99 tunnel construction project, where some lanes and 80 parking spaces are being removed at Pioneer Square.

Workers this week are building detour lanes for waterfront traffic to go under the old Alaskan Way Viaduct, while the state spends up to $500,000 for replacement parking and a marketing campaign, to help merchants through the holiday shopping season.

Ferry-terminal access by car will be more difficult, as will a drive along the waterfront, until the middle of next year.

The changes are needed so tunnel contractors can dig a giant trench between the viaduct and the shoreline. Known as “the box,” it will be reinforced by concrete pilings before the giant boring machine is launched there in 2013. The box will begin near Royal Brougham Way South, but areas near Pioneer Square require ground stabilization and utility relocation. From there, the tunnel machine will dive under the old viaduct foundation and continue below downtown to South Lake Union.

This month, parking spaces were eliminated under the viaduct from South King Street to South Main Street, and the northbound-only driving lane has been narrowed.

A new 100-space parking lot, charging $3 an hour with a four-hour limit, has opened at Pier 48, off Main Street, until Jan. 3. The state Department of Transportation leased the space to Republic Parking to operate the lot to aid shoppers and tourists.

Another 70 short-term parking spaces have been added in the CPS Garage, at First Avenue and Columbia Street, charging $3 an hour.

By early December, a new one-lane detour under the viaduct will extend from Railroad Avenue South — a major route to ferries at Colman Dock — farther north, where it will slant toward Alaskan Way near South Main Street. This will result in a couple of blocks of single-lane road instead of two lanes. Ferry traffic will obstruct other motorists heading north along the waterfront.

The bicycle-pedestrian crossing will remain available at South King Street, heading to the new DOT trail along container-port Terminal 46. Some temporary paved trail will be added under the viaduct.

Upcoming work by City Light will take more parking spaces that are farther north, under the viaduct.

And in the spring, the state will be strengthening the soils and foundations around the old viaduct near Yesler to prevent settlement that could damage the elevated highway — which will operate four more years. When the strengthening and utility work is being done, there will be short-term and partial closures of parking spaces and roadway.

State DOT managers are worried that if cars can’t leave Colman Dock easily entering Seattle, then ferry unloading operations will bog down, and the boat schedules will slip.

The state will add a second northbound lane this spring from Yesler Way to Spring Street, by removing trackway from the abandoned waterfront streetcar. That would make it easier to unload certain ferries so cars can leave either north or south, instead of south only in some cases.

Matt Preedy, deputy administrator for the Highway 99 project, said he is talking with Seattle DOT about changing Yesler Way, so ferry users can drive directly eastbound from Colman Dock into downtown. The state DOT is opening an information office, called Milepost 31,” near the J & M Tavern in Pioneer Square. More information is available at alaskanwayviaduct.com.